Microwave Synthesizers Aid Aircraft Tracking

Significant advances have been made in Laser Detection and Ranging (LADAR) systems, which also are known as Light Detection and Ranging (LIDAR). These advances provide for a broad range of target imaging and tracking applications. LADAR provides significant advantages over conventional RF RADAR in image resolution, as much shorter wavelengths of the electromagnetic spectrum are used (typically in the ultraviolet, visible, or near infrared). Because image resolution depends on the wavelength of the electromagnetic radiation used, shorter wavelengths provide greater resolution of a target's various surfaces. These shorter wavelengths range from about 10 micrometers to the UV (ca. 250 nm).

MIT Lincoln Labs has developed a system that employs a Doppler-based LADAR system. It is used for accurately tracking moving aircraft. The system employs a Kalman-filter target tracker, which is used to provide an accurate estimate of an aircraft's position and velocity. The Kalman filter is a set of mathematical equations that estimate the state of a process. In this case, the Kalman filter is used to determine the position and velocity of an aircraft.

The LADAR system requires two 40-GHz microwave synthesizers. One synthesizer is used to simulate object Doppler shifts by transmitting a variable frequency signal to the receiver. The other synthesizer is used to track the target's Doppler-simulated frequency changes. In addition to the wideband-frequency-generation requirement, the synthesizers must be able to rapidly respond to changes detected by the Doppler Track Kalman filter at the heart of the LADAR system. At the fastest rate, the synthesizers have to be able to frequency sweep at a maximum rate of 500 MHz per second with individual frequency changes occurring as fast as 2 ms.

The Gigatronics Model 2400B Series microwave synthesizer provides the wideband-frequency synthesis and fast frequency switching needed to meet these requirements. The 2400B series provides frequency-switching speeds as fast as 160 µs in List mode. In that mode, the 2400B also is preprogrammed to frequency hop to predefined frequency points.

Clearly, the 160 µs would provide excellent frequency-switching times. Yet the time required to reprogram the list would not allow for fast responses to the changes detected by the Doppler Track Kalman filter. The synthesizer must respond to individual frequency-change commands within a few milliseconds of receiving a remotely controlled command. Conventional ASCII-based General-Purpose-Instrument-Bus (GPIB) transactions can take as long as 20 ms. Both List mode and conventional GPIB communications are prohibitive for such applications. To meet the 2-ms frequencyswitching requirement, the 2400B series synthesizers rely on the Automation Xpress software.

This software includes an application program interface (API) in the form of a Dynamic Link Library (DLL). An API is a set of routines, protocols, and tools for building software applications. The API enables a programmer to individually command frequency changes while taking advantage of the fast-frequency-switching architecture of the 2400B. Automation Xpress significantly reduces the processor burden of the 2400B by transferring the instrument-state processing burden to a PC. Once an instrument-state calculation for generating a frequency is performed, the majority of the time required to switch frequency is the data transfer from the controller to the 2400B.

The switching-time specification for Automation Xpress is 1.0 ms with modern processor and memory configurations (see figure). Typical frequency-switching time—excluding the controller processor overhead—is approximately 1 ms when the GPIB End or Identify signal is used as a starting point for the switching-time measurement to the Lock/Level signal. That signal indicates that the frequency change has been completed.

Operating-system requirements did have to be overcome in order to provide fast switching with Automation Xpress. The LADAR system developed at MIT uses the Linux Fedora Core 4 operating system. In contrast, Automation Xpress was designed for use with the Windows 2000 and XP operating systems. As a result, Gigatronics engineers had to convert the Automation Xpress API to a Linux-based shared library of functions.

The 2400B Series synthesizer has been successfully integrated into the Doppler Track Kalman filter LADAR system designed at MIT. That system is currently undergoing testing.